The principal components of the impeller of an axial flow fan are a support rim or hub, which is rotated about its axis by a motor, and a plurality of blades projecting radially from and carried by the rim. In addition, in some impeller constructions the connection between the inner ends of the blades and the rim permits the pitch of all the blades to be adjusted simultaneously during rotation. It is recognized in the manufacture of impellers that the weight of the blades and of the rim and of the pitch-controlling mechanism when present should be as low as the performance requirements of the fan will permit. For this reason it would be desirable to make the blades of thin material, such as sheet metal, and this can be done when the fan is of relatively small size and need produce only a relatively small pressure increase and a correspondingly small air flow. However, thin blades have not been successful in the past in large high performance fans, due to the inherent problems which a thin blade introduces. As a result, high performance axial flow fan impellers are conventionally fitted with rather thick, heavy blades which are usually cast from metal. These heavy blades in turn require a relatively massive rim to withstand the stresses produced by centrifugal force during running. Further, in fans having blades which can be adjusted during running, the high centrifugal forces resulting from the use of heavy blades place high stresses on the mechanism used for controlling the pitch of the blades.
Considering more in detail the conventional parameters employed in designing an axial flow impeller, it is known that an impeller blade should have a high natural frequency of vibration, i.e. the inherent flexing of the blade about its fixed inner end should take place at high frequency. Since an increase in the thickness of a given blade increases the natural frequency of the blade, one of the first steps in designing an impeller is to determine the minimum thickness required to produce a blade having the necessary high natural frequency. For large fans capable of producing substantial pressure increases and substantial air flows, the minimum blade thickness is relatively large. This, then, is the principal reason for the previously stated requirement to use thick blades. It follows that the thick blades are heavier and that the heavier blades produce greater centrifugal forces on the rim, thereby requiring a rim of greater thickness in the radial direction.
Since the centrifugal force produced by a blade on the rim is many times, for example 1000 times, the weight of the blade, it is evident that if the thickness of the blades can be reduced, several important advantages result, in terms of reduced material and manufacturing costs, reduced fan weight and generally simpler design.
It is also recognized in the art that for a given blade construction, the natural frequency of vibration for that blade places a restraint on the maximum permissible speed for the impeller. It is evident, therefore, that if the natural frequency of the given blades can be increased, the fan can be run at higher speed with a corresponding increase in performance.
The present invention is based on the discovery that the natural frequency of vibration of an axial flow fan blade can be increased by attaching the blade to its rim or hub by a low-friction pivot-like connection which allows swinging movement of the blade relative to the rim. It is now possible, therefore, to reduce the thickness and therefore the weight of the blades, and to restore the resulting reduction in natural frequency by using the pivotal connection. This makes it possible to construct the blades from, for example, sheet metal which can be economically stamped into the desired blade configuration. A reduced blade weight significantly reduces the centrifugal forces on the rim, as discussed above, and therefore the present invention permits the rim to be constructed with a thinner cross section. This not only reduces the weight of the rim, but allows the use of less costly manufacturing techniques.
If the impeller is of the type having controllable-pitch blades, the use of lighter-weight blades reduces the stresses on the control mechanism particularly on the blade thrust bearings. The bearing associated with each blade permits that blade to be turned when the fan is running and must of necessity be constructed to withstand the high stress produced by centrifugal force acting on the blade. The strength of the bearings is often a restraint on the maximum permissible speed of the impeller in a conventional design, but the use of lighter blades, as achieved by the present invention, reduces the stress on the bearings and thereby removes this restraint on speed.
The invention is not, however, limited to impeller constructions in which the blades are of lighter construction than would conventionally be employed. For any given impeller the inclusion of the pivot-like connection between the blade and the rim increases the natural frequency of the blade, and this increases the maximum permissible operating speed of the impeller.
The swinging movement permitted by the pivot-like connection is such that the blade can swing about an axis which is located at the inner end of the blade in a plane perpendicular to a rim radius passing through the connection. In order not to introduce bending forces on the blade, this radius should preferably also pass through the center of gravity of substantially all sections of the blade. The disposition of the axis in the indicated plane is preferably parallel to the average chord of the blade, i.e. a chord which is the average of all chords drawn along the blade from one end to the other, but it may vary considerably from this position. The amount of increased natural frequency of the blade produced by the swinging connection decreases as the angle of the swing axis departs from parallelism with the average blade chord, until essentially no advantage exists when the swing axis is perpendicular to the average blade chord. For most applications the axis should lie within a 40.degree. range in either direction from the average blade chord.
The pivot-like connection may be provided by a variety of different structures several of which are illustrated in the drawings. It is important that the connection avoid pivot structures in which there is sliding contact between parts, as in a conventional hinge, because the friction which friction would develop as a result of centrifugal force on the blade would prevent the free swinging movement required by the present invention.
In its broadest form, the connection may include a universal joint having one end secured to the rim and the opposite end secured to the inner end of the blade, the joint being restrained against turning movement in a pitch-adjusting mode. Alternatively, the connection may be provided by a thin high tensile strength spring type metal band connected between the rim and the inner end of the blade. In a preferred construction, the inner end of the blade carries a pin extending in a plane perpendicular to the axis of the blade. The surface of the pin which faces outwardly toward the blade makes line contact with a surface of a restraining element carried by the rim. The pin can therefore roll against the plate to a limited extent, and as it does so the blade swings. Centrifugal force on the blade forces the pin tightly against the surface, but this does not affect the freedom of the pin to roll and to thereby achieve the desired increase in the natural frequency of the blade. A few degrees of swinging is sufficient, but this small movement must be an unrestrained as possible.